- Add expand_metadata_aliases() for concept fan-out (series→5, author→2,
description→2, tags→3 keys)
- Replace _normalize_metadata_tags in webui/service.py with
normalize_metadata_map from domain
- Replace _normalize_metadata in epub3/exporter.py with
normalize_metadata_map from domain
- Replace manual fan-out in form.py with expand_metadata_aliases()
- Rewrite metadata_overrides.py to use expand_metadata_aliases()
- Remove unused normalize_metadata_casefold import from exporters.py
- 19 new tests for expand_metadata_aliases (1528 total)